The present invention relates to an optical data recording medium for an apparatus operable with optical memories. More particularly, the present invention is concerned with a so-called phase variation type optical data recording medium in which a material constituting a recording layer varies in phase when illuminated by a light beam to thereby allow data to be recorded, reproduced, and rewritten, and a material for a heat-resistant protection layer for the production of the medium.
A recording medium of the type described belongs to a family of conventional optical memory media which selectively allow data to be recorded, reproduced or erased when illuminated by a light beam, particularly a laser beam. The phase variation type recording medium is based on the transition between the crystal phase and the non-crystal phase or between the crystal phases. This type of recording medium has, among others, an overwriting capability using a single beam; it is difficult to provide a magnetooptical memory with this kind of capability. This, coupled with the fact that the recording medium simplifies optics to be built in a drive, is accelerating the study and development of such a recording medium.
U.S. Pat. No. 3,530,441, for example, teaches a phase variation type recording medium using a so-called chalcogen alloy, e.g., Gexe2x80x94Te, Gexe2x80x94Texe2x80x94S, Gexe2x80x94Sexe2x80x94Sn, Gexe2x80x94Texe2x80x94S, Gexe2x80x94Sexe2x80x94Sb, Gexe2x80x94Asxe2x80x94Se, Inxe2x80x94Te or Sexe2x80x94As. To enhance stability and rapid crystallization, Au, Sn and Au, or Pd may be added to Gexe2x80x94Te. The Au, Sn and Au, and Pd addition schemes are respectively disclosed in Japanese Patent Laid-Open Publication Nos. 61-219692, 61-270190, and 62-19490. Further, to improve the repeated recording and erasing ability, Gexe2x80x94Texe2x80x94Sexe2x80x94Sb and Gexe2x80x94Texe2x80x94Sb may each be provided with a particular composition ratio, as proposed in Japanese Patent Laid-Open Publication Nos. 62-73438 and 63-228433. However, none of these prior art schemes satisfies all the characteristics required of the phase variation type rewritable optical memory medium. Particularly, there is a keen demand for measures to improve the recording and erasing sensitivity, to obviate the decrease in erasure ratio ascribable to incomplete erasure at the time of overwriting, and to extend the life of recorded and non-recorded portions.
Japanese Patent Laid-Open Publication No. 63-251290 proposes a recording medium having a recording layer in the form of a single layer of substantially ternary or higher compound. In this document, the single layer of substantially ternary or higher compound refers to a layer containing a compound having a ternary or higher stoichiometric composition (e.g. In3SbTe2) by more than 90 atomic percent inclusive. The above document recites that the recording layer with such a composition improves the recording and erasing characteristic. However, this kind of implementation has a problem that the erasure ratio is low, and a problem that the laser power necessary for recording and erasing data has not been sufficiently lowered yet. Under these circumstances, an optical data recording medium having a high erasure ratio and sensitivity and a desirable repetition characteristic is called for.
In light of the above, some different materials have been developed for a protection layer feasible for a recording medium. For example, there may be used ZnS.SiO2 (Japanese Patent Laid-Open Publication No. 4-74785), SiN or AlN (Japanese Patent Laid-Open Publication No. 63-259855 and Japanese Patent Publication No. 4-74785). However, even with any combination of these materials, it is impossible to satisfy all the characteristics required of the optical recording medium.
It is therefore an object of the present invention to provide a phase variation type data recording medium having a high erasure ratio and allowing data to be repeatedly recorded and erased a number of times by low power even at the time of high-speed recording and erasure, and a material for a heat-resistant protection layer for the production of the medium.
In accordance with the present invention, in a material for a heat-resistant protection layer and constituting one of a plurality of components of a phase variation type recording medium, at least one compound having a thermal conductivity of higher than 10 W/mxc2x7deg inclusive in a bulk state is contained.
Also, in accordance with the present invention, in an optical data recording medium having a substrate and a heat-resistant protection layer, a recording layer and a reflective heat radiation layer sequentially stacked on the substrate, the recording layer mainly consists of Ag, In, Sb and Te, and the heat-resistant protection layer contains at least one compound having a thermal conductivity of higher than 10 W/mxc2x7deg inclusive in a bulk state.